Given that they can't always expect help from the outside, it's easy to see why protestors need a lot of courage and considerable organization to stand up to the guns. The military dictatorship in South Korea cracked down hard on an uprising in the city of Gwangju in 1980, causing hundreds, and perhaps thousands, of casualties. Still, the country's pro-democracy activists successfully demonstrated their mettle in the years that followed, and finally managed to capitalize on national elections in 1988 to usher in genuine democracy. Russian revolutionaries rebounded from the Bloody Sunday killings in 1905 to bring down czarist rule 12 years later. (The Bolsheviks then heaved themselves into power seven months after that.) Still, even these two struggles show just how hard it can be to fight back against a government that's has little compunction about killing its opponents. No one knows that better than the Syrian rebels, whose war against Bashar al-Assad started after his troops viciously crushed peaceful protests in 2011.
Perhaps there is some source of hope to be found in the realization that many of these governments did fall in the end. Of course, every situation is unique, and the "lessons of history" can hardly be considered binding. Egypt faces a period of unparalleled volatility -- and any pundit who claims to know what will happen next is lying. But if history is any guide, we shouldn't expect today's bloodshed to weaken the military's hold any time soon.